The Cascadia subduction zone is where two of Earth's tectonic
plates meet in an epic collision and one haltingly slides below
the other. The Cascadia Fault stretches for almost 700 miles
(1,100 kilometers) from Northern California up to Canada. The
force required to shove a piece of ocean crust into Earth's
mantle can produce megaearthquakes along the zone, as in Japan
and Sumatra.

But unlike its western Pacific cousins, the Cascadia subduction
zone has not experienced a major earthquake since 1700, when an
estimated
9.0-magnitude earthquake generated an enormous tsunami that
killed trees in Puget Sound and traveled across the ocean to
Japan.

The slow-slip and
tremors observed in the area are periodic, coming about every
15 months, said Stanford geophysics professor Paul Segall, and
were first spotted in 2003. The slow-slip earthquakes creep along
the fault at about 4 mph (6.4 kph), for two weeks at a time. The
tremors hum about 18 miles (30 km) below Earth's surface, deeper
than the zone where big earthquakes rupture. Some scientists
think the tremors are evidence of the sinking tectonic plate
slowly dropping into the Earth, which may "load" the shallower,
locked zone of the fault.

Segall's group uses computational models of the region to
determine whether the cumulative effects of many small events can
trigger a major earthquake. The research simulates the
slow-slip and tremors on a computer model of the subduction zone.

Segall noted that the model needs refinement to better match
actual observations on the subduction zone — the decade of
intriguing seismic monitoring records that revealed the tremors.
He hopes to possibly identify the signature of events that could
trigger a large earthquake.

"You have these small events every 15 months or so, and a
magnitude-9 earthquake every 500 years. We need to known whether
you want to raise an alert every time one of these small events
happens," Segall said in a statement.